High performance integrated circuit (IC) packages are well known in the art. Improvements in IC packages are driven by industry demands for increased thermal and electrical performance and decreased size and cost of manufacture.
In general, array packaging such as Plastic Ball Grid Array (PBGA) packages provide a high density of interconnects relative to the surface area of the package. However, typical PBGA packages include a convoluted signal path, giving rise to high impedance and an inefficient thermal path which results in low thermal dissipation performance. With increasing package density, the spreading of heat generated by the device is increasingly important.
Reference is made to FIG. 1 which shows an elevation view of a conventional PBGA package indicated generally by the numeral 20. The PBGA package 20 includes a substrate 22 and a semiconductor die 24 attached to the substrate 22 by a die adhesive. Gold wire bonds electrically connect the die 24 to metal traces on the substrate 22. The wire bonds and die 24 are encapsulated in a molding compound 26. Solder balls 28 are disposed on the bottom surface of the substrate 22 for signal transfer. Because of the absence of a thermal path away from the semiconductor die, thermal dissipation in this package is very poor.
One method of improving heat dissipation is the addition of thermal vias in the substrate. The thermal vias connect the die 24 to some of the solder balls 28 for heat dissipation. While these thermal vias are advantageous for thermal dissipation, the thermal vias are small and increased thermal dissipation in high density packages is still desirable.
Variations to conventional BGA packages have been proposed for the purpose of increasing thermal and electrical performance. FIG. 2 shows an elevation view of a PBGA package of the prior art with a heat sink 30. The heat sink 30 is comprised of a metal plate added to the upper portion of the package 20 for dissipating heat from the upper surface of the package 20. This package still suffers disadvantages, however, as heat must be dissipated from the silicon die 24, through the molding compound 26 and then through the heat sink 30. Thus, heat dissipation away from the silicon die 24 in high density packages is still poor.
FIG. 3 shows an elevation view of yet another variation of the conventional BGA package according to the prior art. This package 20 includes a metal heat spreader 32 that is employed to dissipate heat from the semiconductor die 24 to the surrounding environment. The metal heat spreader 32 includes four legs 36, one leg at each corner. The legs 34 contact ground pads on the substrate 22, thereby providing four contact points with the ground pads. While this package provides better thermal dissipation than the package of FIG. 1, thermal dissipation is still poor and an improved thermal path from the semiconductor die is desirable.
Another example of a variation to conventional BGA packages is described in U.S. Pat. No. 5,977,626, issued Nov. 2, 1999, the contents of which are incorporated herein by reference. The '626 patent discloses a PBGA package having a metal heat spreader in contact with an upper surface of the semiconductor die and ground pads on the substrate. The heat spreader is added to dissipate heat from the semiconductor die to the surrounding environment. These packages also suffer disadvantages, however. One particular disadvantage is that the heat spreader and semiconductor die have significantly different thermo-mechanical properties causing induced stress on the semiconductor die during thermal cycling.
In applicant's own U.S. Pat. No. 6,737,755, issued May 18, 2004, the contents of which incorporated herein by reference, an improved PBGA package is disclosed in which a silicon die adapter is disposed between and fixed to both the semiconductor die and the metal heat spreader. This provides a thermal path from the semiconductor die to the heat spreader. While this package is an improvement over the prior art, further improvements in package reliability while still providing a thermal dissipation path, are still desirable. In particular, reduced interfacial delamination between elements of the package that are fixed together, is desirable.